Copper can be alloyed with refractory metals (RM) such as niobium (Nb), vanadium (V), and chromium (Cr) by consumable electrode melting of composite Cu-RM electrodes. The microstructure of the alloys consists of refractory metal dendrites in a copper matrix. For a description of this kind of alloy, see Downing, Verhoeven and Gibson (1987), J. Appl. Phys., 61:2621-2625.
Cu-RM dendrite-type alloys are quite ductile and may be mechanically reduced to very large drawing strains without breakage. Mechanical reduction, such as by drawing, extrusion, or rolling, converts the RM dendrites into elongated filaments which reinforce and greatly increase the strength of the formed wire, sheet, or other solid configuration.
The consumable electrode arc melting method has also been applied to prepare copper-tantalum (Cu-Ta) alloys, as described in U.S. Pat. Nos. 4,481,030 and 4,600,448. A consumable electrode is prepared with a copper matrix having a plurality of RM strips, such as Nb or Ta, embedded therein. The electrode is subjected to direct current arc melting in an enclosed chamber containing an inert gas (e.g., argon). Reduced gas pressure, such as about two-thirds atmosphere, has been employed. The alloy is formed as the tip of the electrode melts, and the resulting alloy melt is collected in a mold to form an ingot.
This consumable electrode arc melting method was found to be applicable to Nb, V, Cr, and Ta. The process conditions employed were essentially the same, and a sub-atmospheric inert gas pressure was used.
In comparing the alloys produced by the consumable electrode arc melting method, it was found that the strength of the alloys formed into wire increased in relation to the shear modulus of the refractory metal. Cu-Ta alloys had strength superior to Cu-Nb alloys, the shear modulus of Ta being about 1.9 times higher than Nb. This finding suggested the need for alloying copper with refractory metals of much higher shear modulus, in particular with molybdenum (Mo) and tungsten (W). The shear modulus of Mo and W are 3.3 and 4.0 times higher than Nb. Further, such Cu-Mo and Cu-W alloys can be expected to have the same type of phase equilibria with Cu, and the same drawing characteristics as Cu-Nb or Cu-Ta alloys. But despite these expectations, prior to the method improvement of the present invention, it was not found possible to prepare such high strength alloys by the consumable electrode arc melting method.